This invention relates generally to surface acoustic wave (SAW) devices and shallow bulk acoustic wave (SBAW) devices, and more particularly, to low-loss SAW and SBAW filters. Such devices are useful in a variety of applications, including communication systems.
Piezoelectric crystal devices utilizing surface acoustic waves, as contrasted with bulk acoustic waves, have been developed over the last several years. The SAW devices have a number of important advantages over bulk acoustic wave devices, including a higher frequency of operation, and a planar structure that is easy to fabricate and is mechanically rugged.
More recently, another acoustic wave device has been developed, and is sometimes referred to as the shallow bulk acoustic wave (SBAW) device. In this type of device, acoustic waves are propagated at a relatively shallow angle just below the surface of a piezoelectric crystal. A higher velocity of propagation is obtained than in SAW devices, and correspondingly higher frequencies of operation are possible. Alternatively, transducer finger spacings may be made larger for any given frequency. In addition, SBAW devices are less sensitive to surface contamination than SAW devices, and are less responsive to spurious signals of the bulk or SAW type. SBAW devices are described more fully in U.S. Pat. No. 4,349,794 issued in the names of Reynold S. Kagiwada et al.
Acoustic wave devices of both the SAW type and the SBAW type utilize interdigital transducers for converting electrical energy into acoustic or mechanical energy, and vice versa. Basically, these transducers comprise metalized layers formed on the crystal surface in finger-like configurations, like the teeth of a comb. The finger-like elements are usually arranged in two sets, with the fingers in the two sets extending in opposite directions, in an interleaved fashion, from respective elongated pads, known as sum bars.
When an electrical signal is applied to such a transducer, across the sum bars of the two sets of fingers, an acoustic wave is launched in a direction perpendicular to the transducer fingers. Depending on the type of crystal, the crystal cut, and the relative orientation of the transducers, the acoustic wave will be either a surface wave, or a shallow bulk wave, or a combination of the two types. When the acoustic wave encounters a second, similarly structured transducer, it is transformed back into an electricl signal for output from the device. Typically, a transmitting or input transducer in such a device is excited by an oscillatory electrical signal, either in continuous-wave (CW) mode, or in a pulsed mode of operation.
The frequency of operation of SAW and SBAW devices depends largely on the size and geometry of the transducers. Although an electroacoustical transducer will convert an input electrical signal to an acoustic wave of the same frequency, the transducer has a high insertion loss at frequencies outside a band of frequencies determined by the transducer geometry. The transducer operates, in effect, like a bandpass filter, the center frequency of which is determined by the spacing between pairs of transducer fingers, and the pass-band width of which is controllable to some degree by the number of pairs of fingers in the transducer. In general, a transducer with many pairs of fingers will have a narrow-band frequency response, while one with few pairs of fingers will have a wideband frequency response.
A conventional SAW filter device of the type described has an insertion loss of approximately 10-20 decibels (dB), which is too high for many applications of such devices. One reason that the losses are so high is that each transducer is a bidirectional element. When an electrical signal is applied to a conventional interdigital transducer, acoustic waves are launched in two opposite directions. Recently, two different unidirectional types of low-loss SAW filters have been developed with insertion losses of close to 5 dB. The two types, of which the disadvantages will be discussed in more detail, are referred to as the three-phase unidirectional transducer filter and the group-type unidirectional transducer filter.
A three-phase unidirectional transducer comprises three interlaced interdigital transducers to which three phase-spaced input signals are applied. For example, an unshifted input signal is applied to one transducer, the same input signal shifted by 120 degrees is applied to the second transducer, and the same input signal shifted by 240 degrees is applied to the third transducer. The three transducers are aligned on a common axis and are spaced apart to provide a desired reinforcement and cancellation of their acoustic outputs. More specifically, the outputs of the three transducers are in phase in one direction away from the composite set of transducers, but cancel each other in the opposite direction. A similar three-phase transducer is needed to perform the output function. Three-phase transducers not only require the use of 120-degree phase-shifting networks, but also necessitate the use of an airgap crossover to make appropriate connections to the three transducers. This effectively limits the device to lower frequencies, in a range below 500 megahertz (MHz). Another drawback is that the three-phase transducer is difficult to amplitude-weight. It will also be apparent that the three-phase transducer is truly unidirectional only for a particular frequency, since the spacing of the three transducers has to be chosen as a function of wavelength, to obtain the desired reinforcement and cancellation of the signals.
A related approach is the group type unidirectional transducer (GUDT), which uses two interlaced transducers to which input signals are applied ninety degrees out of phase with each other. The spacing between the transducers is such that the acoustic signals from the two reinforce in one direction and cancel in the other. A similar GUDT is used for output purposes. Again, the unidirectional effect is limited to a narrow frequency band, and the GUDT is therefore limited to filter applications requiring higher shape factors and moderate out-of-band rejection. The GUDT is also difficult to amplitude-weight, and requires a meandering ground path to be fabricated on the device.
Yet another approach is to use three bidirectional transducers, arranged along a common axis. When an input signal is applied to the two outer transducers, the overall bidirectional loss is theoretically limited to one half, or 3 dB, since the input energy is split equally at each of the two outer transducers, and one half of the energy is propagated away from the group of three transducers. Ideally, the center transducer receives equal wavefronts from both directions, and the total incident energy may be extracted without significant loss. In practice, however, the center transducer cannot be perfectly matched over the passband of the device, and some of the energy incident on the center transducer is reflected back to the outer transducers. Some of this reflected energy is, in turn, reflected from the outer transducers back to the center transducer again, where it may be transduced into electrical form. This effect is referred to as a triple-transit echo. It manifests itself as a significant passband ripple on the frequency characteristic of the device.
One technique that has been suggested for suppression of the triple-transit echo inherent in the three-transducer approach is to offset one of the outer transducers by one quarter-wavelength with respect to the other, so that there will be a ninety-degree phase difference between the two paths to the center transducer. The signal applied to one of the two outer transducers is also shifted in phase by ninety degrees, to compensate for the offset. Thus, the two primary acoustic signals will arrive at the center transducer exactly in phase, as in the original configuration of the three-transducer filter. However, the signals reflected from the center transducer will subsequently be subject to differing phase shifts. Because of the ninety-degree offset spacing of one of the outer transducers, signals making a triple transit of the device will cancel on return to the center transducer. Unfortunately, this triple-transit suppression technique works effectively only for a single frequency, which is usually selected to be the center frequency of the filter. For most practical filter applications, the triple-transit problem remains and a passband ripple in excess of 1 dB is present in the frequency response.
It will be appreciated from the foregoing that there is still a need for a low-loss acoustic wave filter device that avoids the drawbacks of the prior art. In particular, there is a need for a low-loss acoustic wave device operable over a wide range of frequencies and having having no significant passband ripple in its frequency characteristic. The present invention fulfills this need.